1. Field of the Invention
The present invention relates to air-core gauge driver circuits and, in a preferred embodiment, to air-core gauge control schemes employing pulse width modulated (PWM) signals generated at a variety of frequency rates to compensate for various drive rates of several different gauges.
2. Description of Related Art
An air-core gauge normally has a rotor with a magnetic sensitive region disposed perpendicular to a dial shaft and a pair of stator windings; i.e, a COSINE or (X) winding and a SINE or (Y) winding fixedly arranged at right angles about the shaft. The gauge has a dial and a dial needle. The needle moves over quadrants (and octants of the quadrants) of the dial in, response to movement of the rotor.
Prior dual half-bridge drivers (H-drivers) control the gauge in response to a pair of PWM streams (PWM) and four current-direction signals to provide magnetic fields for rotating the rotor. The current-direction signals select the direction that current passes from each of the H-drives through the gauge windings to place the needle in each of four quadrants of the dial. The PWM signals, first and second positive pulse signals, control the direction and amount of needle movement over the dial within selected quadrants. The PWM signals cause changes in magnitude of the current passing through each winding.
Modulations of the current in the X and Y windings, respectively, vary between 0 and 100%. The modulations vary in proportion to related variations of COSINE and SINE wave function values between 0.degree. and 90.degree.. These function values define the angular position of the needle on the dial. Current in each winding varies in relation to the ON time durations of the modulated pulse stream to produce resultant magnetomotive force (MMF) vectors. The needle positions vary with respect to the vector sum of the SINE and COSINE waves that represent the equivalent % ON time of the PWM signal.
Illustratively, when a PWM signal applied to a Y-winding stays ON 20% of the time, the PWM signal on a X-winding stays ON approximately 97.5% of the time during the same cycle. The current and magnetic fields in the X-winding increase while the current and magnetic fields of the Y-winding decrease causing the rotor to rotate. Rotation continues until magnetic quiescence results between the two fields and the magnetic sensitive region of the rotor. The needle deflects to a location at about 11.25.degree. on the dial. 11.25.degree. corresponds to the vector sum of the COSINE and SINE functions when a 20% duration first PWM signal routes to the X-winding while a 97.5% duration second PWM signal routes to the Y-winding.
If the % ON time of the PWM signal on the X-winding changes to 97.5% while the % ON time of the PWM signal on the Y-winding changes to 20%, the needle seeks a location of about 78.75.degree. on the dial. Of course, the % ON time of the two pulse streams changes back to the original 20:97.5%, or greater, then the quiescent affect of the fields causes the needle to rotate backwards towards the original 11.125.degree. location on the dial.
A problem occurs in the prior push-pull type H-drivers when an attempt to move the needle from, e.g., a 6.4.degree. position to 0.degree. on the dial, or from 83.degree. to 90.degree. on the dial or vice-versa. The needle tends to stick for ratios of 5:99.5% or less. At these ratios, the magnetic field in the dominant winding appears to swamp or dwarf any affects of the magnetic field of the recessive winding. Hence, a quiescent result occurs that seems to bind the magnetic region of the rotor to the dominant field rather than to both fields.
Realizing the needle sticking problem of the push-pull H-driver/air-core gauge circuits, a search for other circuits and schemes to eliminate the problem was initiated. This search resulted in the improved device of the present invention.